<p>Hyperspectral imaging was used to study ontogenetic and heat-induced changes in visible and near-infrared (NIR) reflectance spectra of primary leaves of Hordeum vulgare L. seedlings of different ages. It was found that NIR light reflectance decreased and the mesophyll structure became less compact during primary leaf growing. Chlorophyll pigment absorption increased from 4 to 7 days of growth and then decreased. High temperature (40°C, 3 h, illumination of 120 μmol quanta·m^–2·s^–1) affected insignificantly the reflectance coefficients of leaves of 4- and 7-day-old seedlings in the NIR, but significantly reduced leaf density in 11-day-old plants. At the same time, the absorption of chlorophyll pigments significantly decreased in 4- and 11-day-old seedlings and remained unchanged in 7-day-old leaves after heating. Light absorption by water molecules in the studied seedlings of all age groups did not change significantly after heat exposure. A comparative and correlation analysis of eight commonly used vegetation indices found that exposure to heat led to a significant decrease in the photochemical reflectance index (PRI) in 4-day-old leaves and the red-edge normalized difference vegetation index (NDVI₇₀₅) and structure-insensitive pigment index (SIPI) in 11-day-old seedlings. It was proposed to use these vegetation indices to assess heat stress in plants.</p>